Seamless pipes or tubes are used in many industrial fields such as energy, automobile, chemical, industrial equipment, construction, and the like. Particularly, they are used as an oil well and for transport of crude oil or gas. They play important roles in the fields related to energy resource development in the world.
FIG. 10 schematically shows an example of a representative process of manufacturing seamless pipes or tubes. In FIG. 10, a billet 100 as the material of seamless pipes or tubes is loaded in a rotary hearth type heating furnace 2 and heated. The heated billet 100 is taken out from the furnace 2 and is subjected to piercing in a piercer (hereinbelow, called “piercing mill”) 300, thereby becoming a hollow shell 4. Subsequently, a mandrel bar 5a is inserted into the hollow shell 4 from the rear end, and the hollow shell 4 is elongated by mandrel mill 5 configured by roll stands of five to nine stages to a predetermined dimension.
After that, the mandrel bar in the hollow shell 4 is pulled out, and the hollow shell 4 is hot rolled by a sizing mill 6. The hot rolled pipes or tubes are cooled on a cooling bed 7, cut to the given length, and straightened.
FIG. 11 shows the piercing mill 300 which has a pair of main rolls 111 facing each other in the vertical direction and a pair of disc rolls 112. Both of the rotary axes of the disc rolls 112 are perpendicular to a piercing axis X of the main rolls 111. Both of the main rolls 111 rotate in the same direction. Each of the rotary axes of the main rolls 111 crosses in plan view.
Each guide face 112a of the disc rolls 112 is a curved shape to stably hold a hollow shell. The disc rolls 112 which are disposed near the main rolls 111 guide the billet 100 and hold a hollow shell by the guide faces 112a to keep the shape of a hollow shell.
A cannon 113 which guides the billet 100 at the inlet side of a piercing mill is disposed on the upstream side of the main rolls 111 and a plug bar 114 having an axis same as the piercing axis X1 and supporting a plug at its tip is disposed on the downstream side of those.
As the main rolls 111 rotate in same direction, the billet 100 is fed to the downstream and pierced by the plug bar 114.
As described above, during piercing, the billet 100 or the hollow shell 4 (hereinafter, the billet 100 and the hollow shell 4 are also called as “material”) rotates on the piercing axis X1. On the other hand, the disc rolls 112 rotate on the axis which crosses the piercing axis X1. Consequently, relative slip occurs between disc rolls and material.
In the case of carbon steel, oxide film forms thickly on the material surface by heating in preparation. The oxide film is interposed between the material and the disc rolls 112 during piercing. Consequently, seizure does not occur so much between disc rolls and material.
On the other hand, in the case of high alloy steel such as 13Cr or stainless steel, the oxide film does not form thickly on the material surface, even at high temperature. Therefore, piercing often results in seizure of the billet 100 being pierced and the disc rolls 112. The seizure causes damage on the surface of the hollow shell 4. Additional process such as material grinding of surface, re-polishing of guide face, or attachment/detaching of disc rolls to/from apparatus is required to remove damage caused by seizure. It results in interruption of operation and is consequently disadvantageous from the viewpoint of time and cost.
As a countermeasure against seizure between the material and the disc rolls, a method of spraying lubricant onto the surface of the disc roll guide faces 112a is considered. The “lubricant” herein is different from a normal “lubricant” such as rolling oil, working fluid, or coolant used for reducing friction coefficient. Its principal objective is to prevent seizure. Therefore, in some cases, the friction coefficient between disc roll and material increases.
For example, Patent official gazette 1 discloses a piercing mill 300 in FIG. 11, which has the nozzle 115 for spraying lubricant to main rolls 111 during piecing to prevent slip between the billet 100 and the surface of the main rolls 111. The nozzle is fixed to the end of the cannon 113 at piercing mill side.
Non-patent document 1 discloses lubricant for spraying onto the surface of main rolls, which is made of mostly mixed aqueous solution of boric acid and film forming agent.    Patent official gazette 1: Japanese Patent No. 2,641,834 (p 2, FIGS. 1 and 2)    Non-patent document 1: “Material and process”, The Iron and Steel Institute of Japan, Vol. 8 (1995), p 1218